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Ask the Experts: The exterminator

Every year we are plagued by black flies in our kitchen and conservatory. We once had to get rid of 20 dead flies in a day. The problem stopped over the winter but has now returned. What can we do?

SG, London

Although usually tolerated, houseflies are a significant health risk, and this is the time of year when they start to make their presence felt. Their propensity to transmit disease arises from their mobility and love for dirt — they are as happy in the waste bin as they are on the Sunday joint. Of more concern is the way that they feed. Houseflies cannot eat solids. They vomit digestive juices onto the surface of the food, before sucking (most of it) back up again. They often deposit a dropping before flying off.

There are many types of black fly, and they need different treatments, but, generally speaking, homeowners can